Basic Retrievals in QBE

C.1 Basic Retrievals in QBE

In QBE retrieval queries are specified by filling in one or more rows in the templates of the tables. For a single relation query, we enter either constants or example ele- ments (a QBE term) in the columns of the template of that relation. An example element stands for a domain variable and is specified as an example value preceded by the underscore character (_). Additionally, a P. prefix (called the P dot operator) is entered in certain columns to indicate that we would like to print (or display)

1092 Appendix C Overview of the QBE Language

EMPLOYEE Fname

Super_ssn Dno DEPARTMENT

Minit Lname Ssn

Dname Dnumber Mgr_ssn Mgr_start_date DEPT_LOCATIONS

Dnumber Dlocation PROJECT

Pname Pnumber Plocation

Dnum

WORKS_ON Essn

Pno Hours DEPENDENT

Figure C.1

Essn Dependent_name

The relational schema of Figure 3.5 as it may be displayed by QBE.

values in those columns for our result. The constants specify values that must be exactly matched in those columns.

For example, consider the query Q0 : Retrieve the birth date and address of John B. Smith. In Figures C.2(a) through C.2(d) we show how this query can be specified in

a progressively more terse form in QBE. In Figure C.2(a) an example of an employee is presented as the type of row that we are interested in. By leaving John B. Smith as constants in the Fname , Minit , and Lname columns, we are specifying an exact match in those columns. The rest of the columns are preceded by an underscore indicating that they are domain variables (example elements). The P. prefix is placed in the Bdate and Address columns to indicate that we would like to output value(s) in those columns.

Q0 can be abbreviated as shown in Figure C.2(b). There is no need to specify exam- ple values for columns in which we are not interested. Moreover, because example values are completely arbitrary, we can just specify variable names for them, as shown in Figure C.2(c). Finally, we can also leave out the example values entirely, as shown in Figure C.2(d), and just specify a P . under the columns to be retrieved.

To see how retrieval queries in QBE are similar to the domain relational calculus, compare Figure C.2(d) with Q0 (simplified) in domain calculus as follows:

Q0 : { uv | EMPLOYEE (qrstuvwxyz) and q=‘John’ and r=‘B’ and s=‘Smith’}

Appendix C Overview of the QBE Language 1093

(a)

EMPLOYEE Fname Minit Lname

Salary Super_ssn Dno John

B Smith _123456789 P._9/1/60 P._100 Main, Houston, TX _M _25000 _123456789 _3 (b) EMPLOYEE

Fname Minit Lname

Salary Super_ssn Dno John

B Smith

P._9/1/60 P._100 Main, Houston, TX

(c)

EMPLOYEE Fname Minit Lname

Salary Super_ssn Dno John

B Smith

P._X

P._Y

(d) EMPLOYEE Fname Minit Lname

Salary Super_ssn Dno John

B Smith

P.

P.

Figure C.2

Four ways to specify the query Q0 in QBE.

We can think of each column in a QBE template as an implicit domain variable; hence, Fname corresponds to the domain variable q, Minit corresponds to r, ..., and Dno corresponds to z. In the QBE query, the columns with P. correspond to variables

specified to the left of the bar in domain calculus, whereas the columns with con- stant values correspond to tuple variables with equality selection conditions on them. The condition EMPLOYEE (qrstuvwxyz) and the existential quantifiers are implicit in the QBE query because the template corresponding to the EMPLOYEE relation is used.

In QBE, the user interface first allows the user to choose the tables (relations) needed to formulate a query by displaying a list of all relation names. Then the tem- plates for the chosen relations are displayed. The user moves to the appropriate columns in the templates and specifies the query. Special function keys are provided to move among templates and perform certain functions.

We now give examples to illustrate basic facilities of QBE. Comparison operators other than = (such as > or ≥) may be entered in a column before typing a constant value. For example, the query Q0A : List the social security numbers of employees who work more than 20 hours per week on project number 1 can be specified as shown in Figure C.3(a). For more complex conditions, the user can ask for a condition box, which is created by pressing a particular function key. The user can then type the complex condition. 1

1094 Appendix C Overview of the QBE Language

Figure C.3

WORKS_ON

Specifying complex conditions

(a) Essn

Pno Hours

in QBE. (a) The query Q0A. (b) The query Q0B with a

P.

condition box. (c) The query Q0B without a condition box.

WORKS_ON (b) Essn

Pno Hours

_HX > 20 and (PX = 1 or PX = 2) WORKS_ON

(c) Essn

Pno Hours

For example, the query Q0B : List the social security numbers of employees who work more than 20 hours per week on either project 1 or project 2 can be specified as shown in Figure C.3(b).

Some complex conditions can be specified without a condition box. The rule is that all conditions specified on the same row of a relation template are connected by the and logical connective (all must be satisfied by a selected tuple), whereas conditions specified on distinct rows are connected by or (at least one must be satisfied). Hence, Q0B can also be specified, as shown in Figure C.3(c), by entering two dis- tinct rows in the template.

Now consider query Q0C : List the social security numbers of employees who work on both project 1 and project 2; this cannot be specified as in Figure C.4(a), which lists those who work on either project 1 or project 2. The example variable _ES will bind itself to Essn values in <–, 1, –> tuples as well as to those in <–, 2, –> tuples. Figure C.4(b) shows how to specify Q0C correctly, where the condition ( _EX = _EY ) in the box makes the _EX and _EY variables bind only to identical Essn values.

In general, once a query is specified, the resulting values are displayed in the template under the appropriate columns. If the result contains more rows than can be dis- played on the screen, most QBE implementations have function keys to allow scroll- ing up and down the rows. Similarly, if a template or several templates are too wide to appear on the screen, it is possible to scroll sideways to examine all the templates.

A join operation is specified in QBE by using the same variable 2 in the columns to

be joined. For example, the query Q1 : List the name and address of all employees who

Appendix C Overview of the QBE Language 1095

WORKS_ON

(a)

Essn

Pno Hours

P._ES

2 WORKS_ON

P._ES

(b)

Essn

Pno Hours

1 P._EY Figure C.4 2

P._EX

Specifying EMPLOYEES who work

CONDITIONS

on both projects. (a) Incorrect specification of an AND condition.

_EX = _EY

(b) Correct specification.

work for the ‘Research’ department can be specified as shown in Figure C.5(a). Any number of joins can be specified in a single query. We can also specify a result table to display the result of the join query, as shown in Figure C.5(a); this is needed if the result includes attributes from two or more relations. If no result table is specified, the system provides the query result in the columns of the various relations, which may make it difficult to interpret. Figure C.5(a) also illustrates the feature of QBE for specifying that all attributes of a relation should be retrieved, by placing the P. operator under the relation name in the relation template.

To join a table with itself, we specify different variables to represent the different ref- erences to the table. For example, query Q8 : For each employee retrieve the employee’s

first and last name as well as the first and last name of his or her immediate supervisor can be specified as shown in Figure C.5(b), where the variables starting with E refer to an employee and those starting with S refer to a supervisor.